Aircraft avoidance systems and aircraft warning systems, such as terrain awareness warning systems, are systems used in aircraft, such as fixed wing and rotary wing aircraft, for reducing controlled flight into terrain (herein abbreviated CFIT). CFIT generally refers to an accident whereby an airworthy aircraft inadvertently flies into an obstacle, terrain or water. An aircraft avoidance system operates by providing warnings, either visual, audio or both, to a pilot that his aircraft is in danger of CFIT. Aircraft avoidance systems usually determine the presence of a CFIT danger by at least one of two subsystems, known in the art as ground proximity warning systems (herein abbreviated GPWS) and forward looking terrain avoidance (herein abbreviated FLTA) systems.
In general, FLTA systems operate by executing three major functions. The first function is the definition of a protection envelope around an aircraft. The protection envelope is essentially virtual and defines a volume surrounding the aircraft. Typically, the dimensions of the protection envelope are a function of the aircraft's performance envelope and its flight dynamics during a particular flight. Once a protection envelope has been defined, the second function of FLTA systems is executed. In this function, the protection envelope is constantly compared with terrain elevation data, from a terrain elevation database, at the geographic position of the protection envelope. In other words, the position of the protection envelope at a given moment is compared with terrain elevation data at that given position. The third function of FLTA systems is to issue a warning or an alert to a pilot if the comparison of the second function shows that the protection envelope bisects terrain or an obstacle, or bisects a predetermined safety margin above the terrain or obstacle.
FLTA systems and aircraft avoidance systems are known in the art. In such systems, the protection envelopes are usually defined based on the speed, or velocity, of an aircraft as well as on the predicted flight path of the aircraft. For example, U.S. Pat. No. 6,583,733 to Ishihara, et al., entitled “Apparatus, method and computer program product for helicopter ground proximity warning system” is directed towards a system and method for alerting a pilot of a rotary wing aircraft of the aircraft's proximity to terrain. The method comprises the steps of defining a first set of terrain awareness protection envelopes for alerting a pilot of proximity to terrain during a first set of flight operations and also defining a second set of terrain awareness protection envelopes for alerting the pilot of proximity to terrain during a second set of flight operations. In the method, a signal is received which is indicative of which set of flight operations the aircraft is in. Depending on the signal received, the appropriate protection envelope is calculated dynamically from the first set or second set of terrain awareness protection envelopes. Given the calculated protection envelope, stored terrain data is accessed, and an alert is outputted when the stored terrain data lies within a boundary of the selected protection envelope. The first set of envelopes and the second set of envelopes are determined as a function of a terrain floor boundary and a look ahead distance, with the terrain floor boundary being a function of the speed and altitude of the aircraft.